Currently, there are recordable or write-once-read-many type optical disks such as CD-R/DVD-R and recordable optical disks such as CD-RW/DVD-RW, DVD-RAM, other than a reproduction-only optical disks such as CD and DVD. When recording information onto such recordable optical disks, a reference clock signal for the writing cannot be generated from the reproduction signal used in the conventional reproduction-only disks.
Therefore, the recordable optical disk is constructed as shown in FIG. 8. FIG. 8 shows a part of the surface of DVD-R/RW disk. On the optical disk (hereinafter also referred to as DVD-R/RW) 10, tracks (grooves) 12 which represent a section for recording information are spirally formed in a serpentine (wobbling) fashion with a specific amplitude and cycle. Such a track or groove is referred to as a wobble. When a laser spot 13 is irradiated on the serpentine grooves 12, a light reflected therefrom is detected with an optical detector (not shown) divided into the right and left sides in a push-pull manner, so that a difference of amounts of light can be detected. The signal detected in this manner is referred to as a wobble signal from which the reference clock signal for the writing can be generated.
The grooves 12 are formed in such a manner that a number of pit data 14 representing a variety of information can be written therein, and notched parts called land prepits (hereinafter referred to as LPP) 18 are provided in the land parts 16 between the grooves 12, as also shown in FIG. 9.
In the case of CD-R/RW, the wobbled grooves are also formed as described above. The wobble signal of CD-R/RW is a continuous signal having a frequency of 22.05 kHz±1 kHz at the time of unmultiplied speed. In the case of CD-R/RW, the address information referred to as ATIP (Absolute Time In Pregroove) is superimposed by means of the frequency modulation (FM) on the wobble signal which thereby has a certain frequency bandwidth.
On the other hand, contrary to CD-R/RW, the wobble signal in the case of DVD-R/RW 10 has a single frequency of 140.65 kHz (in the case of unmultiplied speed) with no address information being superimposed thereon, so that the LPP signal obtained from the LPP 18 is used as the address information.
On a portion where the LPP 18 is formed, the land parts on one side are configured to be discontinuous. For this reason, during passage through the discontinuous portion, the amount of light incident upon the optical detectors on both sides of the right and left will steeply change, so that as a result of the push pull, a pulse-form signal will be superimposed as the LPP signal on the wobble signal.
As shown in FIG. 8, a portion (hereinafter referred to as LPP outer peripheral connection part) 18a connected to the outer peripheral side of the groove 12 in the LPP 18 is arranged to overlap with a sufficient accuracy with the maximum amplitude position of the wobble of the groove 12, and to continue to occur for at most three wobbles. A portion (hereinafter referred to as LPP inner peripheral connection part) 18b connected to the inner peripheral side of the groove 12 in the LPP 18 is not always arranged in the maximum amplitude position of the wobble but rather arranged irregularly in such positions as the minimum amplitude position of the wobble or positions between the maximum and minimum amplitudes of the wobble. However, there may be a case where such LPP inner peripheral connection part 18b continues to occur for at most three wobbles.
As a result, the address information is recorded onto DVD-R/RW 10 in combination with the LPP signals for three periods of the LPP outer peripheral connection part 18a. 
In order to extract a wobble signal stably and detect a LPP signal with less mistake from a signal formed by the combination of such wobble and LPP components, it will be required to reduce the leaking of the recorded signal components. In addition, there has been a trend for attaining the high multiple speed of reproduction in DVD-R/RW as well as in CD-R/RW. To this end, it is necessary that the frequency band of LPF (Low Pass Filter) and BPF (Band Pass Filter) and the LPP extraction circuit, which are used in a wobble and LPP signal reproduction circuit, are switched in association with the multiple speed so as to effect the extraction of the wobble and LPP signals.
FIG. 10 shows a construction of the wobble and LPP signal reproduction circuit employed in the conventional optical disk device. In the following, the wobble and LPP signal reproduction circuit may be abbreviated to the reproduction circuit.
The reproduction circuit 20 is constructed to comprise a quadripartite sensor 21, operational amplifiers 23, 24, AGCs (Auto Gain Controllers) 31, 32 to which input and output sides HPFs (High Pass Filters) 26, 27, 28, and 29 are connected, a gain variable differential amplifier 34 with a frequency variable LPF, an over level limiter 36, a BPF 38, a level hold section 40, a DAC (Digital Analog Converter) 42, an adder 44, an AGC 49 to which input and output sides HPFs 46, 47 are connected, and binary comparators 51, 52.
When the laser spot 13 is irradiated on the optical disk 10 as shown in FIG. 8, the reflected light will form a spot image on the quadripartite sensor 21 four-divided in a manner as shown by A, B, C, and D in FIG. 10. Here, the quadripartite sensor 21 first takes up signals A, B, C and D and then adds them in the circumferential direction to generate signals (A+D) and (B+C) in order to reproduce the wobble and LPP signals. The signal (A+D) is a signal on the left side (AD side) of the advancing direction of the groove 12 shown by an arrow head Y1 in the Figure, and the signal (B+C) is a signal on the right side (BC side).
The signals (A+D) and (B+C) are inputted into the HPFs 26, 27 in which their DC offset components are removed, and thereafter inputted into the AGCs 31, 32. In the AGCs 31, 32, in order to remove RF components and in-phase noise components, the amplitudes of both signals (A+D) and (B+C) are aligned. The signals (A+D) and (B+C), of which amplitudes have been adjusted to be constant, are inputted into the differential amplifier 34 by way of the HPFs 28, 29.
In the differential amplifier 34, the difference between the signals (A+D) and (B+C) is taken and processed by the frequency variable LPF with a predetermined gain, so as to be formed as a differential signal DIFO.
In processing of the differential amplifier 34, the difference between the signals (A+D) and (B+C) is taken to cancel out the pit data 14, and the gain for the wobble and LPP signals included in each of the signals (A+D) and (B+C) is increased so as to amplify the wobble and LPP signals.
The differential signal DIFO outputted from the differential amplifier 34 will include only the wobble signal component after the LPP signal component is removed by the over level limiter 36. This wobble signal 37 is outputted to the level hold section 40 and the HPF 46 through the BPF 38. Then, a DC offset component and a noise component, etc. included in the wobble signal are removed while passing from the HPF 46 via the AGC 49 and through the HPF 47, and the wobble signal is formed to have a constant amplitude. The wobble signal 54 is thereby reproduced.
The wobble signal 54 is inputted into the binary comparator 51 and is binarized by the comparison with a slice level V1, so as to be formed as a binary wobble signal 56.
A limit value SLL of the over level limiter 36 described above is generated as follows. That is, the peak and bottom levels of the output of the BPF 38 are held (peak held and bottom held) in the level hold section 40. A predetermined DC offset voltage outputted from the DAC 42 is added to each of the hold values by means of the adder 44, as a result of which the limit value SLL is generated.
The limit value SLL is also used as a slice level V2 of the binary comparator 52. That is, the slice level V2 is formed as the peak hold value or the bottom hold value, to each of which the offset voltage has been added. They are referred to as the peak hold value and the bottom hold value in the following, and it is assumed that each of them has been added with the offset voltage.
The slice level V2 is compared in the binary comparator 52 with the differential signal DIFO, as a result of which the binarized LPP signal 58 is reproduced. Whether, in the case of the reproduction, the peak hold value or the bottom hold value is used as the slice level V2 which is inputted into the binary comparator 52, depends upon the polarity of the LPP component included in the differential signal DIFO. For example, the peak hold value PL is used for the LPP signals P1, P2 due to the LPP outer peripheral connection part 18a, because the signals appear in upward projecting pulse state, as shown in FIG. 11A. However, the bottom hold value BL is used for the LPP signals P3, P4 due to the LPP inner peripheral connection part 18b, because the signals appear in downward projecting pulse state when the signals are taken up with the opposite polarity.
That is, as shown in FIG. 11A, the pulse state LPP signals P1, P2 superimposed on the differential signal DIFO can be removed by means of the peak hold value PL in the over level limiter 36. On the other hand, the pulse state LPP signals P3, P4 superimposed on the differential signal DIFO can be removed by means of the bottom hold value BL. With such operation, the wobble signal 37, as shown in FIG. 11B, is outputted from the over level limiter 36.
In the binary comparator 52, as shown in FIG. 11A, the differential signal DIFO is compared with the peak hold value PL for detecting the LPP signals P1, P2 which exceed the peak hold value PL, so as to form the detected signals to be binary LPP signals 58. A conventional optical disk device of this type is disclosed in Japanese Patent Laid-Open No. 7-296395 and No. 2002-216363.
However, the prior art optical disk device has the following problems. In the case where the LPP inner peripheral connection part 18b of the optical disk 10 is positioned in the vicinity of the center of the amplitude of the wobble, as shown in FIG. 11A, the LPP signals P3, P4 from the LPP inner peripheral connection part 18b in the vicinity of the center thereof will project from the vicinity of the center of the amplitude C1 of the differential signal DIFO. Even if the LPP signals P3, P4 are removed with the bottom hold value BL, some parts of the LPP signal components P3, P4 cannot be removed, as shown by P3a, P4a in FIG. 11B.
Such LPP signal components P3a, P4a could have a negative effect on the wobble signal 37, and cause a jitter to occur in which, when the wobble signal 54 from the AGC 49 is binarized by the binary comparator 51, the resultant signal fluctuates along the time axis. That is, the jitter performance will be deteriorated.
In addition, when the optimal slice level V2 for removing the LPP signals due to the LPP outer peripheral connection part 18a from the differential signal DIFO is varied due to the amplitude fluctuation of the wobble signal, for example, when the slice level V2 based on the peak hold value PL as shown in FIG. 11B is varied, a part of the LPP signal component P1, as shown by the reference character P1a, can be left unremoved. The fact that such LPP signal component P1a is left unremoved and the amount of such unremoved component is varied will also result in the deterioration of the jitter performance. Moreover, the LPP signal component P1a left unremoved may also cause the amplitude of the wobble signal to change in a stage subsequent to the BPF 38.
If, in the case of reproducing the LPP signal 58, a pulse component P5 which projects from the peak hold value PL (slice level V2) exists in the wobble signal component due to a noise or the fluctuation of the wobble signal component, as shown in FIG. 11A, a maldetection will occur in which the pulse P5 is detected as the LPP signal 58a, as shown in FIG. 11C.
In order to prevent such maldetection of the LPP signal, in the conventional optical disk device, it is necessary to set the limit value SLL used as the slice level V2 to a value with a certain margin. However, with such margin, the LPP signal component P1a left unremoved will also be left in the wobble signal, thereby deteriorating the jitter performance. A method of separating the slice level V2 from the limit value SLL of the over level limiter 36 may be considered, but such method will cause the circuit scale to be increased.
In particular, in the optical disk 10 which has been already recorded, if RF components are included in each of the signals A, B, C and D due to the difference in the reflection factor between the mark part 12a and the non-mark part 12b, which parts represent the pit data 14 shown in FIG. 8, the amplitude of the wobble and LPP signals will be changed by a noise due to the RF components and the presence or the absence of the marks, so that the optimal limit value SLL cannot be obtained. As a result, the slice level V2 will be also changed.
Because of the reasons such as described above, it has been difficult to reproduce properly the wobble signal 54, the binary wobble signal 56, and the LPP signal 58.
The invention is based on the recognition of the above-mentioned problems, and aims at providing a slew rate limiter and an optical disk device in which the wobble and LPP signals can be properly reproduced without the need of increasing the circuit scale.